Combustion of C₁ and C₂ PFAS: Kinetic modeling and experiments

Figures & data

Figure 1. EPA’s pilot-scale “Rainbow” research furnace with temperature and calculated residence time profiles at 45 kW natural gas, 20% excess air used for model conditions. FTIR samples were collected at Port 18. Note the 1 m length scale. The Air Pollution Control System (APCS) consists of a facility afterburner, temperature quench, baghouse, and NaOH wet scrubber.

Figure 2. Temperature profile vs. calculated residence time for the Rainbow furnace at 20% excess air. Residence times correspond to the measured temperature profile used for the model (45 kW load) and the 64 kW experiment. These temperatures were obtained by suction pyrometer. Temperatures for the 40 and 45 kW experiments (solid symbols) are from bare thermocouple furnace centerline measurements taken immediately before PFAS injection at corresponding model port locations.

Table 1. Compounds measured by FTIR in the present study

Fluorinated species		Non-fluorinated species
		Carbon dioxide	CO2
Difluoromethane	CH2F2	Carbon monoxide	CO
Carbon tetrafluoride	CF4	Ethane	C2H6
Carbonyl fluoride	CF2O	Ethylene	C2H4
Fluoromethane	CH3F	Formaldehyde	CH2O
Hexafluoroethane	C2F6	Methane	CH4
Hydrogen fluoride	HF	Nitric oxide	NO
Sulfur hexafluoride^1	SF6	Nitrogen dioxide	NO2
Trifluoromethane	CHF3	Nitrous oxide	N2O
		Water	H2O

¹SF₆ is a potential tracer compound.

Table 2. Comparison of measured (Exp) and calculated (Model) PFAS destruction efficiencies (DEs) within the Rainbow furnace as a function of compound and injection location. Measurements performed by FTIR at Port 18 (see Figure 2)

PFAS injection location	CF4 DE (%)				CHF3 DE (%)			C2F6 DE (%)
	Exp 1 40 kW	Exp 2 45 kW	Exp 3 64 kW	Model 45 kW	Exp 1 40 kW	Exp 2 45 kW	Model 45 kW	Exp 1 40 kW	Exp 2 45 kW	Model 45 kW
Natural gas	58.5	89.5	94.9	97.6	-	>99^4	>99.9	>99^4	>99^4	>99.9
Combust air	--	82.6	88.7	n/a^1	--	>99^4	n/a^1	--	>99^4	n/a^1
Port 4	--	13.7	--	~0^2	>99^4	>99^4	>99.9	>99^4	>99^4	>99.9
Port 6	--	12.9	--	~0^2	--	>99^4	>99.9	--	>99^4	99.5
Port 8	--	11.7	--	~0^2	--	>99^4	>99.9	78.2	>99^4	63.9
Port 10	--	12.5	--	~0^2	>99^4	>99^4	>99.9	25.5	>99^4	8.0
Port 11	8.2	--	--	--	--	--	--	--	--	--
Port 12	--	--	--	--	94.3	>99^4	98.5	~0^3	86.2	~0^2

¹The PFR model assumes perfect mixing of reactants, separate PFAS introduction with the combustion air is not applicable (n/a). The model used an initial adiabatic flame temperature (2071°C) and a linear temperature decay to Port 1, after which the measured temperature profile was used.

²Due to numerical rounding in the model the calculated DE was slightly negative.

³Due to measurement uncertainty, the calculated DE was slightly negative.

⁴Measured FTIR concentration of target analyte was below the ASTM D6248 MDC3 value and considered non-detect.

Table 3. Comparison of measured (Exp) and calculated (Model) PFAS PICs (including residual PFAS and HF, in bold italic font) from the Rainbow furnace as a function of compound and injection location. Measurements performed by FTIR at Port 18 (see Figure 2). Measured species were >3x the SLC residuals and are shown to the nearest 10 ppbv with ± one standard deviation. Modeled species were >10 pptv, 45 kW furnace load

PFAS Injection location	CF4 (ppmv)			CHF3 (ppmv)			C2F6 (ppmv)
	Exp 1 40 kW	Exp 2 45 kW	Model 45 kW	Exp 1 40 kW	Exp 2 45 kW	Model 45 kW	Exp 1 40 kW	Exp 2 45 kW	Model 45 kW
Natural gas	CF4 – 12.53 ± 0.08 HF – 28.73 ± 0.88	CF4 – 3.57 ± 0.06 HF – 101.72 ± 0.48	CF4 – 0.80800 HF −134.00 CF2O – 0.00020	--	CHF3 – ND HF – 92.79 ± 0.46	CHF3 – ND HF – 103.00	C2F6 – ND HF – 143.99 ± 1.40	C2F6 – ND HF – 212.25 ± 0.70	C2F6 – ND HF – 205.00 CF4 – 0.00001 C3H6 – 0.00031
Combustion air	--	CF4 − 5.95 ± 0.04 HF – 88.32 ± 0.34	--	--	CHF3 – ND HF – 85.90 ± 0.41	--	--	C2F6 – ND HF – 196.00 ± 1.51	--
Port 4	--	CF4 − 29.36 ± 0.28 HF – 2.69 ± 0.01	CF4 – 34.200 HF – 0.01600 CF2O – 0.00133	CHF3 – ND HF – 52.25 ± 0.66	CHF3 – ND HF – 63.81 ± 0.53 CF4 – 8.24 ± 0.18	CHF3 – ND HF – 99.200 CF2O – 1.7200	C2F6 – ND HF – 191.42 ± 0.44 CF4 – 0.76 ± 0.003	C2F6 – ND HF – 84.26 ± 0.24 CF4 – 30.59 ± 0.18	C2F6 – ND HF – 175.00 CF4 – 0.01150 CF2O – 14.90
Port 6	--	CF4 – 29.58 ± 0.15 HF – 2.41 ± 0.01	CF4 – 34.200 HF – 0.00118 CF2O – 0.00002	--	CHF3 – ND HF – 85.09 ± 3.27 CF4 – 2.82 ± 0.05	CHF3 – ND HF – 95.500 C2F6 – 0.00001 CF2O – 3.5600	--	C2F6 – ND HF – 194.90 ± 0.87 CF4 – 5.95 ± 0.02	C2F6 – 0.15900 HF – 145.00 CHF3 – 0.00009 CF4 – 0.01660 CF2O – 29.900
Port 8	--	CF4 – 29.97 ± 0.11 HF – 2.42 ± 0.01	CF4 – 34.200 HF – 0.00011 CF2O – 0.00018	--	CHF3 – ND HF – 96.38 ± 0.69 CF4 – 0.29 ± 0.001	CHF3 – ND HF – 91.900 C2F6 – 0.00131 CF2O – 5.3500	C2F6 – 6.29 ± 1.20 HF – 114.01 ± 14.59	C2F6 – ND HF – 222.88 ± 1.85 CF4 – 0.93 ± 0.02	C2F6 – 12.100 HF – 77.500 CHF3 – 0.00594 CF4 – 0.02020 CF2O – 27.600
Port 10	--	CF4 – 29.99 ± 0.24 HF – 2.59 ± 0.005	CF4 – 34.22 HF – ND	CHF3 – ND HF – 59.72 ± 4.23 C2F6 – 0.93 ± 0.12	CHF3 – ND HF – 99.58 ± 0.23	CHF3 – 0.00001 HF – 86.367 CF4 – 0.00001 CF2O – 8.0700 C2F6 – 0.01990 CF3COF – 0.00038	C2F6 – 21.36 ± 0.21 HF – 25.25 ± 0.16	C2F6 – ND HF – 228.11 ± 2.37 CF4 – 0.21 ± 0.004	C2F6 – 30.800 HF – 10.500 CHF3 – 0.01430 CF4 – 0.00725 CF2O – 4.8800
Port 11	CF4 − 27.73 ± 0.33 HF – 1.90 ± 0.02 C2F6 – 0.11 ± 0.002	--	--	--	--	--	--	--	--
Port 12	--	--	--	CHF3 – 1.63 ± 0.04 HF – 48.57 ± 2.25 C2F6 – 0.98 ± 0.02	CHF3 – ND HF – 97.90 ± 1.25 C2F6 – 0.26 ± 0.02	CHF3 – 0.49200 HF – 77.800 CF4 – 0.00003 C2F6 – 0.08310 CF2O – 11.400 CHFO – 0.00006 CHF2CF3 – 0.00001 C2F4 – 0.00002 CF3COF – 0.00079 CF3CHO – 0.00002	C2F6 – 30.86 ± 0.53 HF – 2.87 ± 0.06	C2F6 – 4.90 ± 0.07 HF – 192.56 ± 2.90	C2F6 – 34.000 HF – 0.69700 CHF3 – 0.01580 CF4 – 0.00091 CF2O – 0.39100

Figure 3. Real-time FTIR species concentrations measured at Port 18 depicting CHF3 injection at Ports 10 and 12 (45 kW load). In this case, CHF3 was introduced, and after a brief transitory period stable FTIR traces are achieved. Note the absence of detectable CHF3, CF4 formation as a PIC at both Port 10 and Port 12, and C2F6 formation as a PIC during Port 12 injection. CF4 concentrations (above) were <3x the CLS residual value and therefore, not reported in Table 3.